Abstract: Advanced reprocessing processes require not only the recovery of all uranium and plutonium from the spent nuclear fuel, but also the separation and recovery of some key fission products, such as ¹³⁷Cs and ⁹⁰Sr. “Fresh” spent nuclear fuel contains several Cs isotopes, such as ¹³³Cs, ¹³⁴Cs, ¹³⁵Cs, ¹³⁶Cs and ¹³⁷Cs. ¹³⁴Cs and ¹³⁶Cs are mainly derived from neutron activation reactions, which are usually not the focus of attention because of their small amounts and short half-lives. ¹³³Cs, ¹³⁵Cs and ¹³⁷Cs are mainly generated from fission reactions, and their cumulative fission yields are all around 6%. Among them, ¹³³Cs is a stable nuclide, while ¹³⁵Cs has an extremely long half-life (about 2.3 million years). On the other hand, ¹³⁷Cs (t_1/2=30.2 a) is known mainly for its versatility. After the separation of Pu and U from the spent nuclear fuel with the plutonium uranium reduction extraction (Purex) process, radioactive Cs⁺ exists in the Purex raffinate with other fission products and minor actinides. 1,3-alternate calix4arene-crown-6 ligands are among the best ligands regarding Cs⁺ separation from such kind of high-level liquid waste (HLLW). Many investigations were carried out to develop separation processes based on those ligands. In order to further improve ligand solubility in organic phase, reduce the loss of the organic phase, and avoid the occurrence of the third phase during the extraction process, many efforots have been made to tune the organic diluents and to modify of the molecular structures of the ligands. Since substituent groups on 1,3-alternate calix4arene-2,4-crown-6 have important effects on Cs⁺ extraction, it is possible to improve the extraction performance by using new substituents. In this study, a new ligand, i.e., 1,3-di(4-tert-butylbenzyloxy)calix4arene-(4-tert-butyl)benzo-crown-6 (L1), was synthesized and characterized. The extraction mechanism, extraction selectivity and complexation of L1 to Cs⁺ were investigated by liquid-liquid extraction, MS analysis of the loading organic phase, UV spectroscopy titration and IR spectroscopy. It is found that the solvent extraction of Cs⁺ with L1 in n-octanol reaches equilibrium within 5 min. The ratio of L1 to Cs⁺ in the extracted complex is 1∶1. The apparent equilibrium constant of the solvent extraction reaction is lg K_ex=3.31±0.07. The solvent extraction reaction is exothermic and the enthalpy change is −54.0 kJ/mol. The results of solvent extraction from simulated HLLW evidence the good selectivity of L1 to Cs⁺ over other coexisting fission products. In addition, L1 exhibits higher Cs⁺ distribution coefficient and separation factor (SF_Cs/M) than 1,3-dioctyloxycalix4arene-crown-6 (L2), implying a promising application prospect.